Retrograde signaling from organelles to the nucleus is an important part of the cellular signaling network and has been implicated in cell aging and tumor progression. This work plans to achieve a deeper understanding of these signaling pathways by investigating how plant chloroplasts send signals to the nucleus. Recently it has been shown that chlorophyll intermediates Protoporphyrin IX and Mg-Protoporphyrin IX, synthesized in the chloroplast, are signal inputs for at least two distinct pathways that regulate gene expression in the nucleus (phytochrome and GUN signaling pathways, respectively). Using the flowering plant Arabidopsis thaliana as a model system, the proposed studies will integrate genetic, molecular, as well as biochemical approaches to complete the following specific aims; (1) elucidate the mechanism(s) by which the accumulation of Mg-Protoporphyrin IX in the chloroplast regulates gene expression in the nucleus; (2) and determine the role of Protoporphyrin IX in phytochrome signaling and how its accumulation regulates nuclear gene expression. Completion of the proposed projects will contribute to the understanding of intracellular signaling, organelle communication, and how metabolic pathways and light can regulate gene expression.